The present invention relates to a motion conversion device that is capable of variably changing a transmission gear ratio and is superior in conversion efficiency, a flexible actuator using such a device, and a joint driving unit.
In recent years, there have been strong expectations for robots that are operated in an area close to the person, such as medical-use robots, home-use robots, and job-assistant robots for use in factories. Unlike industrial robots, it is important for these robots to ensure safety when made in contact with the person. In order to suppress an impact upon contact, a force to be exerted on the contact point needs to be minimized. Accordingly, a torque in the joint needs to be controlled so as to make the joint flexible for the robot arm. However, a force (torque) control operation by using an actuator for driving the joint fails to infinitely increase a response frequency, with the result that, in a case when a force in a high-frequency region is exerted (for example, when a robot arm collides with the person), it is not possible to perform an appropriate control operation. Normally, a joint driving mechanism uses a combination of a motor and a speed reducer, and the inertia for the robot arm is indicated by a value obtained by multiplying the inherent inertia of the motor by a square of a speed reducing ratio. For this reason, in a state where the force control is not effectively exerted, an extremely great force is exerted onto the contact point, with the result that the safety is not sufficiently ensured by relying only upon the force control.
In view of this issue, a system has been proposed in which an actuator and a load are connected to each other by using an elastic member referred to as Series Elastic Actuators (SEA) (for example, see Patent Document 1: U.S. Pat. No. 5,650,704). Since even a force in a high-frequency range that cannot be controlled by the actuators is constrained by the flexibility of the elastic member, the SEA are flexible actuators that can always realize a flexible joint for the arm and more highly safe operations can be ensured. In contrast, since the SEA and a load are connected to each other through the elastic member, a controllable frequency band is lowered in comparison with the conventional system. In order to compensate for these disadvantages, a system referred to as “Distributed macro-mini actuation (DM2)” in which an actuator for use in high frequencies is additionally installed (for example, see Non-Patent Document 1: IEEE Robotics & Automation Magazine, Volume 11, Issue 2, 12 to 21 pages, published in June, 2004), and a system referred to as “Variable Stiffness Transmission (VST)” in which the rigidity of the elastic member is variably changed (for example, see Non-Patent Document 2: IEEE Robotics & Automation Magazine, Volume 11, Issue 2, 22 to 33 pages, published in June, 2004) have been proposed.
In order to improve operation efficiency while maintaining flexibility, it will be desirable to provide a structure in which a force of the elastic structural member having a potential energy increased in response to a displacement is connected to an external load through a transmission capable of variably changing a transmission gear ratio with a reverse input being available. In this structure, a generated force of the elastic structural member is converted to an arbitrary force that is connected to an external load, and driving and regenerating operations can be switched without intermittences depending on operation directions. However, in a case of rocking operations in a joint driving process of a robot arm, there is a demand to the transmission for changing the transmission gear ratio with a small displacement, and there is also a demand for transmitting an output with a high force (torque)/velocity ratio. Under these conditions, even in a case when an actuator is structured by using a generally-used friction-type continuously variable transmission as a continuously variable transmission, a great energy is required for altering the transmission gear ratio. Therefore, it has been difficult to achieve an efficient flexible actuator.
In view of the above, an object of the present invention is to provide a motion conversion device that is capable of variably changing a transmission gear ratio and is superior in conversion efficiency, as well as a flexible actuator using such a device, and a joint driving unit.